refactor: message.content always returns a string (#72)

Not to sure but this could be a simple fix for system messages having "no message content".

Everytime someone uses the `client.on("message",...)` function they have to include a check if `message.content` even exists when wanting to work with `message.content` since if you don't the program breaks every time it recieves a system message, which returns a message with undefined content. In my opinion making `message.content` always return a string is a better alternative to undefined. It would make using revolt.js more convenient for most of the developers.

src/classes/Message.ts

@@ -113,7 +113,7 @@ export class Message {
    * Content
    */
   get content() {
-    return this.#collection.getUnderlyingObject(this.id).content;
+    return this.#collection.getUnderlyingObject(this.id).content ?? "";
   }
 
   /**